The present invention is directed to methods for detecting or measuring Notch activation by observing or measuring the appearance of Notch on the cell surface or by observing or measuring Notch cleavage products that are indicative of Notch activation. The present invention is also directed to methods for detecting a molecule that modulates Notch activation by observing or measuring a change in the amount of Notch expressed on the cell surface or a change in the amount or pattern of Notch cleavage products. The present invention is also directed to a substantially purified activated heterodimeric form of Notch and pharmaceutical compositions and kits thereof.
Genetic and molecular studies have led to the identification of a group of genes which define distinct elements of the Notch signaling pathway. While the identification of these various elements has come exclusively from Drosophila using genetic tools as the initial guide, subsequent analyses have lead to the identification of homologous proteins in vertebrate species including humans. FIG. 1 depicts the molecular relationships between the known Notch pathway elements as well as their subcellular localization (Artavanis-Tsakonas et al., 1995, Science 268:225-232).
The Drosophila Notch gene encodes an xcx9c300 kD transmembrane protein that acts as a receptor in a cell-cell signaling mechanism controlling cell fate decisions throughout development (reviewed, e.g., in Artavanis-Tsakonas et al., 1995, Science 268:225-232). Closely related homologs of Drosophila Notch have been isolated from a number of vertebrate species, including humans, with multiple paralogs representing the single Drosophila gene in vertebrate genomes. The isolation of cDNA clones encoding the C-terminus of a human Notch paralog, originally termed hN, has been reported (Stifani et al., 1992, Nature Genetics 2:119-127). The encoded protein is designated human Notch2 because of its close relationship to the Notch2 proteins found in other species (Weinmaster et al., 1992, Development 116:931-941). The hallmark Notch2 structures are common to all the Notch-related proteins, including, in the extracellular domain, a stretch of 34 to 36 tandem Epidermal Growth Factor-like (EGF) repeats and three Lin-12/Notch repeats (LN repeats), and, in the intracellular domain, 6 Ankyrin repeats and a PEST-containing region. Like Drosophila Notch and the related C. elegans genes lin-12 and glp-1 (Sternberg, 1993, Current Biology 3:763-765; Greenwald, 1994, Current Opinion in Genetics and Development 4:556-562), the vertebrate Notch homologs play a role in a variety of developmental processes by controlling cell fate decisions (reviewed, e.g., in Blaumueller and Artavanis-Tsakonas, 1997, Persp. on Dev. Neurobiol. 4:325-343). (For further human Notch sequences, see International Publication WO 92/19734.)
The extracellular domain of Notch carries 36 Epidermal Growth Factor-like (EGF) repeats, two of which (repeats 11 and 12) have been implicated in interactions with the Notch ligands Serrate and Delta. Delta and Serrate are membrane bound ligands with EGF homologous extracellular domains, which interact physically with Notch on adjacent cells to trigger signaling.
Functional analyses involving the expression of truncated forms of the Notch receptor have indicated that receptor activation depends on the six cdc10/ankyrin repeats in the intracellular domain. Deltex and Suppressor of Hairless, whose over-expression results in an apparent activation of the pathway, associate with those repeats.
Deltex is a cytoplasmic protein which contains a ring zinc finger. Suppressor of Hairless on the other hand, is the Drosophila homologue of CBF1, a mammalian DNA binding protein involved in the Epstein-Barr virus-induced immortalization of B cells. It has been demonstrated that, at least in cultured cells, Suppressor of Hairless associates with the cdc10/ankyrin repeats in the cytoplasm and translocates into the nucleus upon the interaction of the Notch receptor with its ligand Delta on adjacent cells (Fortini and Artavanis, 1994, Cell 79:273-282). The association of Hairless, a novel nuclear protein, with Suppressor of Hairless has been documented using the yeast two hybrid system; therefore, it is believed that the involvement of Suppressor of Hairless in transcription is modulated by Hairless (Brou et al., 1994, Genes Dev. 8:2491; Knust et al. 1992, Genetics 129:803).
Finally, it is known that Notch signaling results in the activation of at least certain basic helix-loop-helix (bHLH) genes within the Enhancer of Split complex (Delidakis et al ., 1991, Genetics 129:803). Mastermind encodes a novel ubiquitous nuclear protein whose relationship to Notch signaling remains unclear but is involved in the Notch pathway as shown by genetic analysis (Smoller et al., 1990, Genes Dev. 4:1688).
The generality of the Notch pathway manifests itself at different levels. At the genetic level, many mutations exist which affect the development of a very broad spectrum of cell types in Drosophila. Knockout mutations in mice are embryonic lethals consistent with a fundamental role for Notch function (Swiatek et al., 1994, Genes Dev. 8:707). Mutations in the Notch pathway in the hematopoietic system in humans are associated with lymphoblastic leukemia (Ellison et al., 1991, Cell 66:649-661). Finally the expression of mutant forms of Notch in developing Xenopus embryos interferes profoundly with normal development (Coffman et al., 1993, Cell 73:659). Increased level of Notch expression is found in some malignant tissue in humans (International Publication Wo 94/07474).
The expression patterns of Notch in the Drosophila embryo are complex and dynamic. The Notch protein is broadly expressed in the early embryo, and subsequently becomes restricted to uncommitted or proliferative groups of cells as development proceeds. In the adult, expression persists in the regenerating tissues of the ovaries and testes (reviewed in Fortini et al., 1993, Cell 75:1245-1247; Jan et al., 1993, Proc. Natl. Acad. Sci. USA 90:8305-8307; Sternberg, 1993, Curr. Biol. 3:763-765; Greenwald, 1994, Curr. Opin. Genet. Dev. 4:556-562; Artavanis-Tsakonas et al., 1995, Science 268:225-232). Studies of the expression of Notch1, one of three known vertebrate homologues of Notch, in zebrafish and Xenopus, have shown that the general patterns are similar; with Notch expression associated in general with non-terminally differentiated, proliferative cell populations. Tissues with high expression levels include the developing brain, eye and neural tube (Coffman et al., 1990, Science 249:1438-1441; Bierkamp et al., 1993, Mech. Dev. 43:87-100). While studies in mammals have shown the expression of the corresponding Notch homologues to begin later in development, the proteins are expressed in dynamic patterns in tissues undergoing cell fate determination or rapid proliferation (Weinmaster et al., 1991, Development 113:199-205; Reaume et al., 1992, Dev. Biol. 154:377-387; Stifani et al., 1992, Nature Genet. 2:119-127; Weinmaster et al., 1992, Development 116:931-941; Kopan et al., 1993, J. Cell Biol. 121:631-641; Lardelli et al., 1993, Exp. Cell Res. 204;364-372; Lardelli et al., 1994, Mech. Dev. 46:123-136; Henrique et al., 1995, Nature 375:787-790; Horvitz et al., 1991, Nature 351:535-541; Franco del Amo et al., 1992, Development 115:737-744). Among the tissues in which mammalian Notch homologues are first expressed are the pre-somitic mesoderm and the developing neuroepithelium of the embryo. In the pre-somitic mesoderm, expression of Notch1 is seen in all of the migrated mesoderm, and a particularly dense band is seen at the anterior edge of pre-somitic mesoderm. This expression has been shown to decrease once the somites have formed, indicating a role for Notch in the differentiation of somatic precursor cells (Reaume et al., 1992, Dev. Biol. 154:377-387; Horvitz et al., 1991, Nature 351:535-541). Similar expression patterns are seen for mouse Delta (Simske et al., 1995, Nature 375;142-145).
Within the developing mammalian nervous system, expression patterns of Notch homologue have been shown to be prominent in particular regions of the ventricular zone of the spinal cord, as well as in components of the peripheral nervous system, in an overlapping but non-identical pattern. Notch expression in the nervous system appears to be limited to regions of cellular proliferation, and is absent from nearby populations of recently differentiated cells (Weinmaster et al., 1991, Development 113:199-205; Reaume et al., 1992, Dev. Biol. 154:377-387; Weinmaster et al., 1992, Development 116:931-941; Kopan et al., 1993, J. Cell Biol. 121:631-641; Lardelli et al., 1993, Exp. Cell Res. 204:364-372; Lardelli et al., 1994, Mech. Dev. 46:123-136; Henrique et al., 1995, Nature 375:787-790; Horvitz et al., 1991, Nature 351:535-541). A rat Notch ligand is also expressed within the developing spinal cord, in distinct bands of the ventricular zone that overlap with the expression domains of the Notch genes. The spatio-temporal expression pattern of this ligand correlates well with the patterns of cells committing to spinal cord neuronal fates, which demonstrates the usefulness of Notch as a marker of populations of cells for neuronal fates (Henrique et al., 1995, Nature 375:787-790). This has also been suggested for vertebrate Delta homologues, whose expression domains also overlap with those of Notch1 (Larsson et al., 1994, Genomics 24:253-258; Fortini et al., 1993, Nature 365:555-557; Simske et al., 1995, Nature 375:142-145). In the cases of the Xenopus and chicken homologues, Delta is actually expressed only in scattered cells within the Notch1 expression domain, as would be expected from the lateral specification model, and these patterns xe2x80x9cforeshadowxe2x80x9d future patterns of neuronal differentiation (Larsson et al., 1994, Genomics 24:253-258; Fortini et al., 1993, Nature 365:555-557).
Other vertebrate studies of particular interest have focused on the expression of Notch homologues in developing sensory structures, including the retina, hair follicles and tooth buds. In the case of the Xenopus retina, Notch1 is expressed in the undifferentiated cells of the central marginal zone and central retina (Coffman et al., 1990, Science 249:1439-1441; Mango et al., 1991, Nature 352:811-815). Studies in the rat have also demonstrated an association of Notch1 with differentiating cells in the developing retina have been interpreted to suggest that Notch1 plays a role in successive cell fate choices in this tissue (Lyman et al., 1993, Proc. Natl. Acad. Sci. USA 90:10395-10399).
A detailed analysis of mouse Notch1 expression in the regenerating matrix cells of hair follicles was undertaken to examine the potential participation of Notch proteins in epithelial/mesenchymal inductive interactions (Franco del Amo et al., 1992, Development 115:737-744). Such a role had originally been suggested for Notch1 based on the its expression in rat whiskers and tooth buds (Weinmaster et al., 1991, Development 113:199-205). Notch1 expression was instead found to be limited to subsets of non-mitotic, differentiating cells that are not subject to epithelial/mesenchymal interactions, a finding that is consistent with Notch expression elsewhere.
Expression studies of Notch proteins in human tissue and cell lines have also been reported. The aberrant expression of a truncated Notch1 RNA in human T-cell leukemia results from a translocation with a breakpoint in Notch1 (Ellisen et al., 1991, Cell 66:649-661). A study of human Notch1 expression during hematopoiesis has suggested a role for Notch1 in the early differentiation of T-cell precursors (Mango et al., 1994, Development 120:2305-2315). Additional studies of human Notch1 and Notch2 expression have been performed on adult tissue sections including both normal and neoplastic cervical and colon tissue. Notch1 and Notch2 appear to be expressed in overlapping patterns in differentiating populations of cells within squamous epithelia of normal tissues that have been examined and are clearly not expressed in normal columnar epithelia, except in some of the precursor cells. Both proteins are expressed in neoplasias, in cases ranging from relatively benign squamous metaplasias to cancerous invasive adenocarcinomas in which columnar epithelia are replaced by these tumors (Mello et al., 1994, Cell 77:95-106).
Insight into the developmental role and the general nature of Notch signaling has emerged from studies with truncated, constitutively activated forms of Notch in several species. These recombinantly engineered Notch forms, which lack extracellular ligand-binding domains, resemble the naturally occurring oncogenic variants of mammalian Notch proteins and are constitutively activated using phenotypic criteria (Greenwald, 1994, Curr. Opin. Genet. Dev. 4:556; Fortini et al., 1993, Nature 365:555-557; Coffman et al., 1993, Cell 73:659-671; Struhl et al., 1993, Cell 69:1073; Rebay et al., 1993, Cell 74:319-329; Kopan et al., 1994, Development 120:2385; Roehl et al., 1993, Nature 364:632).
Ubiquitous expression of activated Notch in the Drosophila embryo suppresses neuroblast segregation without impairing epidermal differentiation (Struhl et al., 1993, Cell 69:331; Rebay et al., 1993, Cell 74:319-329).
Persistent expression of activated Notch in developing imaginal epithelia likewise results in an overproduction of epidermis at the expense of neural structures (Struhl et al., 1993, Cell 69:331).
Neuroblast segregation occurs in temporal waves that are delayed but not prevented by transient expression of activated Notch in the embryo (Struhl et al., 1993, Cell 69:331).
Transient expression in well-defined cells of the Drosophila eye imaginal disc causes the cells to ignore their normal inductive cues and to adopt alternative cell fates (Fortini et al., 1993, Nature 365:555-557).
Studies utilizing transient expression of activated Notch in either the Drosophila embryo or the eye disc indicate that once Notch signaling activity has subsided, cells may recover and differentiate properly or respond to later developmental cues (Fortini et al., 1993, Nature 365:555-557; Struhl et al., 1993, Cell 69:331).
For a general review on the Notch pathway and Notch signaling, see Artavanis-Tsakonas et al., 1995, Science 268:225-232.
Ligands, cytoplasmic effectors and nuclear elements of Notch signaling have been identified in Drosophila, and vertebrate counterparts have also been cloned (reviewed in Artavanis-Tsakonas et al., 1995, Science 268:225-232). While protein interactions between the various elements have been documented, the biochemical nature of Notch signaling remains elusive. Expression of truncated forms of Notch reveal that Notch proteins without transmembrane and extracellular domains are translocated to the nucleus both in transgenic flies and in transfected mammalian or Drosophila cells (Lieber et al., 1993, Genes and Development 7:1949-1965; Fortini et al., 1993, Nature 365:555-557; Ahmad et al., 1995, Mechanisms of Development 53:78-85; Zagouras et al., 1995, Proc. Natl. Acad. Sci. USA 92:6414-6418). Sequence comparisons between mammalian and Drosophila Notch molecules, along with deletion analysis, have found two nuclear localization sequences that reside on either side of the Ankyrin repeats (Stifani et al., 1992, Nature Genetics 2:119-127; Lieber et al., 1993, Genes and Development 7:1949-1965; Kopan et al., 1994, Development 120:2385-2396). These findings prompted the speculation that Notch may be directly participating in nuclear events by means of a proteolytic cleavage and subsequent translocation of the intracellular fragment into the nucleus. However, conclusive functional evidence for such a hypothesis remains elusive (Artavanis-Tsakonas et al., 1995, Science 268:225-232).
Citation or identification of any reference in Section 2 or any other section of this application shall not be construed as an admission that such reference is available as prior art to the present invention.
The present invention is directed to methods for detecting or measuring Notch activation by observing or measuring the appearance of Notch on the cell surface or by observing or measuring Notch cleavage products that are indicative of Notch activation. In one aspect of this embodiment of the invention, the method for detecting or measuring Notch activation in a cell comprises detecting or measuring the expression of Notch on the surface of said cell, wherein the presence and amount of Notch on the surface indicates the presence and amount, respectively, of Notch activation. In another aspect, the method comprises detecting or measuring the expression of one or more Notch cleavage products selected from the group consisting of NEC and NTM. In yet another aspect, the method comprises detecting or measuring one or more fragments of Notch selected from the group consisting of an amino-terminal fragment of full-length Notch terminating between the epidermal growth factor-like repeat domain and the transmembrane domain (in particular, between the Lin-12/Notch repeats and the transmembrane domain) of full-length Notch, and a carboxy-terminal fragment of full-length Notch with its amino terminus situated between the epidermal growth factor-like repeat domain and the transmembrane domain (in particular, between the Lin-12/Notch repeats and the transmembrane domain), or detecting or measuring one or more fragments of Notch selected from the group consisting of Notch fragments having a molecular weight of about 270, 200, 170, 140, 110, 100, 90 and 85 kilodaltons. In yet another aspect, the method comprises detecting or measuring a Notch heterodimer containing a reducing agent-sensitive linkage.
The present invention is based, at least in part, on the discovery that Notch in its active form, i.e., the form that mediates signal transduction and that binds Notch ligands such as Delta, is a heterodimer of two Notch cleavage products, an about (xc2x110%) 180 kilodaltons (kDa) subunit (NEC) and an about (xc2x110%) 110 kDa subunit (NTM), which are tethered together through a reducing agent-sensitive linkage. Full length Notch is not expressed on the cell surface and is ligand inaccessible. As shown by way of example infra, the two subunits arise due to a proteolytic cleavage of the full length Notch molecule in the trans-Golgi at a site in Notch amino-terminal to the transmembrane domain and carboxy-terminal to the EGF repeat region, thus generating an extracellular fragment (NEC) of about 180 kDa and a transmembrane/intracellular fragment (NTM) of about 110 kDa. The detection of full length Notch and of Notch cleavage products, as well as Notch that is present on the cell surface, can be carried out by methods well known to those of skill in the art, e.g., precipitation or binding to an immobilized binding partner (e.g., on a plate or column), e.g., anti-Notch antibodies or ligands of Notch, such as Delta and Serrate.
The detection or measurement of Notch activation is important in the study and manipulation of differentiation processes, since Notch plays a key role in cell fate (differentiation) determination. Also, disorders of cell fate, in particular hyperproliferative (e.g., cancer) or hypoproliferative disorders, involving aberrant or undesirable levels of active Notch expression can be diagnosed or screened for by detecting such active Notch expression, as described more fully infra. Molecules that modulate Notch function are important tools for studying and manipulating differentiation processes, e.g., in expanding cell populations without substantial differentiation (International Publication WO 97/11716), in cancer studies and therapy (International Publication WO 94/07474), and differentiation studies on normal tissue.
In another embodiment, the present invention is also directed to methods for identifying a molecule that modulates Notch activation by detecting or measuring a change in the amount of Notch expressed on the cell surface or a change in the amount or pattern of Notch cleavage products. In one aspect of this embodiment of the invention, the method for identifying a modulator of Notch activation comprises providing a cell with a candidate modulator molecule and detecting or measuring the amount of Notch on the surface of the cell, in which a difference in the presence or amount compared to a cell not contacted with the candidate molecule indicates that the candidate molecule modulates Notch activation. In another aspect, the method for identifying a modulator of Notch activation comprises providing a cell with a candidate modulator molecule and detecting or measuring the expression by the cell of one or more Notch cleavage products selected from the group consisting of NEC and NTM, in which a difference in the presence or amount of said one or more cleavage products compared to a Notch cell not contacted with the candidate molecule indicates that the molecule modulates Notch activity.
In an alternative aspect, the method for identifying a modulator of Notch activation comprises contacting a candidate modulator molecule with a full length Notch in the presence of a composition comprising cellular proteins, under conditions conducive to cleavage of the full-length Notch by one or more components of the composition and detecting or measuring the amount of Notch cleavage products NEC and NTM that result, in which a difference in the presence or amount of said Notch cleavage products compared to a full-length Notch in presence of said composition not contacted with the candidate molecule indicates that the molecule modulates Notch activity.
The present invention is also directed to a substantially purified active form of Notch which comprises Notch fragments tethered together through a reducing agent-sensitive linkage, and pharmaceutical compositions and kits thereof.